Airplane physics question

Somehow, that part of the question never shows up on my screen...

 

I would like to thank the silly Russian pilot @ :30 for solving this puzzle. Yes, the plane can take off. And quite handily (if it's an SU-33).



Note that the wheels are spinning the same speed as the runway is moving, yet the plane is able to skid the wheels and take off. Just as I had theorized.

Nothing more to see here folks.

 

The conveyor belt designed to exactly match wheel speed moving in the opposite direction of rotation would effectively cancel out wheel rotation as the craft gains airspeed, or depending on your interpretation, accelerate it exponentially. Either way, a conveyor cannot hold an aircraft stationary no matter what it is doing.

So, plane at 50mph ground speed equals zero wheel speed as the conveyor matches craft airspeed to cancel out wheel rotation/matched speed.

Plane on a conveyor that isn't moving flies. Conveyor matches airspeed, flies. Conveyor moving in opposite direction of aircraft at any speed, aircraft flies, (as long as it takes off before the tires explode from centripetal force).

The only force it could impart on the craft is acceleration of the wheels/tires opposite of intended direction which is inconsequential to anything that doesn't require 100% of available power in ideal conditions to fly.

 

You guys serious. What if the. Plane was on ice and had ice skates instead of wheels. And zero friction.

 

The sequence starts when the plane powers up the engine(s). The plane forces the wheels to start to spin. The conveyor spools up to match the speed of the wheels. However, because the plane is moving forward, the wheels are always trying to spin slightly faster than the conveyor. This forces the conveyor to accelerate continuously, until the tires reach their limit, and begin to skid. The plane moves forward with the tires in a spinning skid, like a drag racer doing a burnout.

This would probably be an easier takeoff, than simply trying to takeoff with the brakes locked.

 

With the tires on the aircraft free wheeling why would they break traction with the conveyor?

Either way I agree, still easier than locked brakes.

 

The tire/wheel/hub assembly will have a certain mass. We know from Newton's laws, that mass will want to stay at rest.


Same reason free wheeling tires skid when landing.

 

If the massive conveyor belt/travelator type arrangement brings it up to speed to develop lift, without wind it will be dead weight and never leave the ground.

 

Not the same at all. Tires that are not spinning that touch down at 150+mph of course will break traction. Tires on a conveyor both starting at zero speed that is far less likely. If the conveyor accelerated that fast the tires would likely just instantly explode.

I still think the conveyor would just cancel out and zero wheel speed while craft gains airspeed. It is stated that conveyor matches wheel speed in opposite direction of rotation. Matched but spinning opposite direction would equal zero wheel speed. Matched but turning same direction (not what is stated) would accelerate the wheels exponentially.

mcmimo- the conveyor cannot influence the plane, not enough to make it stay on the ground. If you did change the scenario to your example however the plane would fly. Locked tires and accelerating conveyor would push the craft through the air until it lifted off. (briefly unless it continued under it's own power). Conveyor moving doesn't bring the atmosphere with it.

 

The "touch down" tires skid for the same reason. Because the mass wants to remain at rest, pitted against the runway/conveyor relative motion. Would a skidding tire explode ? Maybe not. Those SU-33 didn't look to explode, even though skidding on the same flat spot. I have seen burnout contests where the drivers spin the wheels until failure, and it does take some time before the tires eventually fail. Of course that is car tires vs. aircraft tires. But I suspect the aircraft tires are quite rugged.

The original question posed about this simply asked if a plane could take off from a giant conveyor moving in the opposite direction. The answer is yes, because the forward motion of the airplane is generated by moving air, not spinning the wheels. Someone later added this requirement for the conveyor to match the speed of the wheels. The only way for the plane to take off under those conditions, would be to skid the tires. If the tires were rolling at takeoff with the plane moving forward, they would always necessarily be spinning faster than the conveyor.

 

The touch down tires do want to remain at rest. At touch down they are instantaneously accelerated to 150+mph (minus loss of traction) at a time when full weight is not on them. Instant of touch down is the start of weight transfer to the ground.

In the conveyor example, both the tires and conveyor start at rest so there's no 150+mph speed differential that is at first connected immediately and lightly.

So you have the full weight of the craft on the tires with nothing spinning.

Since the wheels are not driven and 'free wheel' the only thing resisting their rotation is polar moment of inertia.

The traction between the two surfaces is much too hard to overcome when the only resistance is accelerating the mass of the wheel/tire assemblies.

In order to break traction in this example the conveyor would need to accelerate so quickly and violently that tire acceleration would instantly be out of control.

It's not as if as the plane reaches 10mph airspeed you would be able to break traction between the tire and conveyor without massive acceleration. Like pulling a table cloth out from under a glass. Must be quick or the glass follows the table cloth.

Only way your example could work is if the brakes on the aircraft were dragging enough to cause a break in traction but not enough so they could no longer spin at all.

Still, the example states the conveyor matches the speed of the wheels moving in the opposite direction of rotation. That equals the conveyor just zeroing out wheel speed as the craft gains airspeed.

If the example stated it matches the speed of the wheels moving in the same direction of rotation that would be an example of the conveyor accelerating the wheel/tire assemblies.

Hey, at least we agree it would fly

 

Yes, if you are a running person or if you are riding a bike or motorcycle on it. All of which are driven by the connection of said example to ground surface.

Does not apply to a thrust driven aircraft. A conveyor cannot hold it stationary.

Get on your treadmill wearing a pair of roller skates and squeeze the trigger on a fire extinguisher aimed ahead of you.

See how much of a stationary device your treadmill is now that you are thrust driven

 

Interesting thought.

But I think if the conveyor was moving forward at the same speed as the plane, the wheels would not turn at all. If the conveyor moved forward faster than the plane, the wheels would be turning backwards, but this would not help the plane take off any faster. Only if the conveyor was moving forward faster than the plane's normal take off speed, and the plane had the brakes applied would the plane take off any faster. The conveyor at that point would be working like a "mass diver".

https://en.wikipedia.org/wiki/Mass_driver

 

Yes agreed, if the conveyor was moving forward at the same speed as the plane the wheels would not turn at all. That follows the criteria of - match the speed of the wheels at any given time, moving in the opposite direction of rotation.

As in, if the airplane reached 100mph airspeed, the wheels forward motion and the conveyors forward motion are also both 100mph, even if the wheels are not rotating. (as it did say match speed not rpm)

 

How is this thread still going? Planes don't transmit power to the pavement like a car, motorcycle, or person would. It's all thrust instead. That's how planes can takeoff with pontoons (regardless of water movement like a river), skis, or wheels. It doesn't matter what is happening on the surface to a plane...that's not how it flies. It's all airspeed.

 

Pavement! We walk on those in the UK unless it's moving and then it's a conveyor [of people]

 

No matter how many times it is explained there are some people that just cannot grasp this.

I don't know why it bothers me so much. It is so simple. When people argue to the contrary I think it would be less painful to smash my head against a brick wall than read another response stating "it's on a conveyor it can't move" Aaaaaaaaah!!!!!

If I was Rob I would permaban everyone who voted -no-.

 

A plane taking off from a conveyor is simple.

However, the requirement to match the speed of the conveyor to the wheels of the plane is an engineering impossibility. Therefore, the question contains a faulty premise. Because of the forward motion of the plane, the wheels will always be moving faster than the conveyor. The conveyor will forever be trying to catch up. Excepting the case of skidding tires.

 

Disagree.

Unless you can tell me why post #2564 does not meet all criteria.

 

Criteria states "moving in the opposite direction of rotation". Referring to the wheels. Therefore, the wheels must have rotation. In your example, there would be no rotation of the wheels. At least, that's the way I understood the question.

 

This is how I understand it, from my perspective.

If a wheel was hovering above the conveyor at 10mph ground speed and the conveyor was moving in the same direction of rotation at 10mph, you could drop the wheel on the conveyor and they would be matching speed.

If a wheel was hovering above the conveyor at 10mph ground speed and the conveyor was moving 10mph in the opposite direction of the tire, when you drop the tire on the conveyor it would equal a burnout at a combined speed of 20mph.

That being said and this being re-stated- "designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation"

It makes sense to me that as the plane gains airspeed if the conveyor moved in the opposite direction of rotation and exactly matching the speed of the wheels, the conveyor would perfectly match airspeed, cancelling out wheel rotation, meeting all criteria.

Again, it states it is "moving in the opposite direction of rotation", which it is, even with no wheel speed. A minimum wheel speed is not mentioned.

Either way, whether or not the question is asked correctly or even possible as asked does not really bother me. The 22 votes to that do not bother me. It's the 147 no votes that bother me.

Which is, the argument and belief that an actual plane parked on an actual conveyor could be held stationary by said conveyor.

 

If you really insist on wheel speed matching, there are planes that can takeoff with locked (non-rotating) wheels and skidding (on a non-moving runway). I believe the Mig29 and Su27 are among those that can.

 

Yes I know, and have provided an example (see post 2552), the SU-33. I suspect most jet powered aircraft could do this. A Rotax powered ultralight ? Maybe not.

 

Incorrect.

If the conveyor matched the speed of the wheels, there would be no forward motion, excepting a skid scenario.

IDK what the numbers would be for a particular plane, but let's say plane X's wheels need to spin at 2,000 RPM in order for it to take off on a normal runway. Now we put plane X on a conveyor that is spinning at 2,000 RPM in the opposite direction of take off. Does this stop the plane from moving forward and taking off ? No. The plane takes off, but the wheels are now spinning at 4,000 RPM instead of 2,000 RPM. The plane's wheels must be moving faster than the conveyor's spin in order for it to move forward.

This is why so many people are thinking that the plane will not take off. Because if the wheels spin at the same RPM as the conveyor's movement, there is no forward motion. However, this thinking is neglecting the possibility of a skidding take off.

 

Just because you don't understand what I'm saying doesn't mean it's incorrect.

My example is the airspeed of the wheels. If the plane is going 100mph in relation to the ground, the wheels are going 100mph in relation to the ground and the conveyor is going 100mph in relation to the ground. This is all happening with no wheel rotation.